In a conventional image pickup apparatus having an image pickup device structured by a plurality of pixels having different photoelectric conversion characteristics different from each other on each side of the inflection point, for example, an image pickup device (hereinafter, referred to as linear log sensor) having a linear photoelectric conversion characteristic on the low illumination intensity side and a logarithmic photoelectric conversion characteristic (hereinafter, referred to as linear log characteristic) on the high illumination intensity side, due to characteristic variations of the device structured by the pixels, the inflection point may be varied and the image quality is adversely affected to no small extent.
Therefore, as an inflection point variation correction method, a method for storing the photoelectric conversion characteristic of all the pixels and the standard photoelectric conversion characteristic and correcting imaged image data of all the pixels to data fitted to the standard photoelectric conversion characteristic is proposed (for example, refer to Japanese Unexamined Laid-Open Patent Publication H11-298799). Further, in the Japanese Laid-Open Patent Publication, a method for storing the ratio of the temperature coefficient to the ambient temperature of all the pixels to the standard temperature coefficient and correcting the temperature characteristic on the basis of the ambient temperature detected by the temperature sensor is described.
Furthermore, a method for correcting a signal offset in the logarithmic characteristic area to an arbitrary offset, then correcting a signal in the linear characteristic area, and detecting the temperature of the image pickup device and correcting the temperature is proposed (for example, refer to Japanese Unexamined Laid-Open Patent Publication 2000-175108).
However, in the method of Japanese Unexamined Laid-Open Patent Publication H11-298799, for example, in the photoelectric conversion characteristic of all the pixels of the image pickup device structured by 2000000 pixels used in an inexpensive digital camera, to divide the overall illumination intensity area into 1000 parts and store each of them as 16-bit (2-byte) data, a huge memory exceeding a personal computer 2,000,000×1000×2=4 giga bytes long is necessary and it cannot be applied to an image pickup apparatus such as a digital camera. Further, to compare the image data of all the pixels with the stored photoelectric conversion characteristic and correct the image data to data fitted to the standard photoelectric conversion characteristic from the results thereof, very many calculations are necessary, and a lot of time is taken in the calculation, so that it cannot be applied to an image pickup apparatus such as a digital camera.
Further, in the method of Japanese Unexamined Laid-Open Patent Publication 2000-175108, for example, in the two photoelectric conversion characteristic graphs with the inflection points shifted (inflection points 901d and 903d) shown in FIG. 16, when intending to allow one photoelectric conversion characteristic 903 (having a linear characteristic 901a and a logarithmic characteristic 903c bounded by each other with the inflection point 903d) to execute a parallel movement 905 in the direction of the axis of ordinate of the drawing so as to fit it to another standard characteristic 901 (having the linear characteristic 901a and logarithmic characteristic 903c bounded by each other with the inflection point 901d), the part of the logarithmic characteristic 903c having illumination intensity of the image pickup surface higher than Lm coincides with the logarithmic characteristic 901c, though a part 903b of the logarithmic characteristic 903c having illumination intensity of the image pickup surface lower than Lm becomes a logarithmic characteristic 907b and does not coincide with a part 901b of the linear characteristic 901a having illumination intensity of the image pickup surface higher than Ln. Namely, the shifts of the inflection points cannot be corrected, and the error between the two photoelectric conversion characteristics cannot be corrected. Here, FIG. 16 is a schematic view showing the two photoelectric conversion characteristic graphs with the inflection points shifted (the inflection points 901d and 903d), and the axis of abscissa is a logarithmic axis indicating illumination intensity L of the image pickup surface, and the axis of ordinate is a linear axis indicating photoelectric conversion output VP, and the illumination intensity of the image pickup surface at the inflection point 901d is Lm, and the photoelectric conversion output is Vthm, and the illumination intensity of the image pickup surface at the inflection point 903d is Ln, and the photoelectric conversion output is Vthn.